The invention relates to a method of detecting aberrations of an optical imaging system, comprising the steps of:
arranging a test object in the object plane of the system;
providing a photoresist layer in the image plane of the system;
imaging the test object by means of the system and an imaging beam;
developing the photoresist layer, and
detecting the developed image by means of a scanning detection device having a resolution which is considerably larger than that of the imaging system.
The fact that the resolution of the scanning detection device is considerably larger than that of the imaging system means that the detection device allows observation of details which are considerably smaller than the details that can still be separately imaged by the imaging system.
An optical imaging system in the form of a projection lens system having a large number of lens elements is used in photolithographic projection apparatuses which are known as wafer steppers or as wafer step-and-scanners. Such apparatuses are used, inter alia, for manufacturing integrated circuits, or ICs. In a photolithographic projection apparatus, a mask pattern present in the mask is imaged a large number of times, each time on a different area (IC area) of the substrate by means of a projection beam having a wavelength of, for example, 365 nm in the UV range, or a wavelength of, for example, 248 nm in the deep UV range, and by means of the projection lens system.
The method mentioned above is known from the opening paragraph of EP-A 0 849 638, relating to a method of measuring the comatic aberration of projection lens systems in lithographic projection apparatuses.
The aim is to integrate an ever-increasing number of electronic components in an IC. To realize this, it is desirable to increase the surface area of an IC and to decrease the size of the components. For the projection lens system, this means that both the image field and the resolution must be increased, so that increasingly smaller details, or line widths, can be imaged in a well-defined way in an increasingly larger image field. This requires a projection lens system, which must comply with very stringent quality requirements. Despite the great care with which such a projection lens system has been designed and the great extent of accuracy with which the system is manufactured, such a system may still exhibit aberrations such as spherical aberration, coma and astigmatism which are not admissible for the envisaged application. In practice, a lithographic projection lens system is thus not an ideal, diffraction-limited system but an aberration-limited system. Said aberrations are dependent on the positions in the image field and are an important source of variations of the imaged line widths occurring across the image field. When novel techniques are used to enhance the resolving power, or the resolution, of a lithographic projection apparatus, such as the use of phase-shifting masks, as described in, for example, U.S. Pat. No. 5,217,831, or when applying an off-axis illumination as described in, for example, U.S. Pat. No. 5,367,404, the influence of the aberrations on the imaged line widths still increases.
Moreover, the aberrations are not constant in modem lithographic projection lens systems. To minimize low-order aberrations, such as distortion, curvature of the field, astigmatism, coma and spherical aberration, these systems comprise one or more movable lens elements. The wavelength of the projection beam or the height of the mask table may be adjustable for the same purpose. When these adjusting facilities are used, other and smaller aberrations are introduced. Moreover, since the intensity of the projection beam must be as large as possible, lithographic projection lens systems are subject to aging so that the extent of the aberrations may change with respect to time.
Based on the considerations described above, there is an increasing need for a reliable and accurate method of measuring aberrations.
It has also been proposed to use for the projection beam a beam of extreme UV (EUV) radiation, i.e. radiation at a wavelength in the range of several um to several tens of nm. The resolution of the projection lens system can thereby be enhanced considerably without increasing the numerical aperture (NA) of the system. Since no suitable lens material is available for EUV radiation, a mirror projection system instead of a lens projection system must then be used. A lithographic mirror projection system is described in, inter alia, EP-A 0 779 258. For reasons analogous to those for the lens projection system, there is a need for an accurate and reliable method of measuring aberrations for this EUV mirror projection system as well.
The opening paragraph of said EP-A 0 849 638 rejects the method in which the image of a test mask formed in the photoresist layer is scanned with a scanning detection device in the form of a scanning electron microscope. Instead, it is proposed to detect said image with optical means. To this end, a test mask having one or more patterns of strips which are alternately radiation-transmissive and radiation-obstructive, i.e. an amplitude structure, is used. The comatic aberration of a projection system can be detected with such a pattern. The detection is based on measuring the widths of the light or dark strips in the image formed and/or measuring the asymmetry between the strips at the ends of the image of the patterns.
It is an object of the present invention to provide a method of the type described in the opening paragraph, which is based on a different principle and with which different aberrations can be measured independently of each other. This object is met by a method which comprises the steps of:
arranging a test object, which comprises at least one closed single figure having a phase structure, in the object plane of the system;
providing a photoresist layer in the image plane of the system;
imaging the test object by means of the system and an imaging beam;
developing the photoresist layer;
observing the developed image by means of a scanning detection device having a resolution which is considerably larger than that of the imaging system;
subjecting the observed image to a Fourier analysis in order to ascertain at least one of different types of changes of shape in the image of the single figure, each type of shape change being indicative of a given kind of aberration, which is represented by a specific Fourier harmonic being a combination of a number of Zernike polynominals each preceded by a weighting factor, the measurement of the Zernike coefficients being carried out by the steps:
setting at least one of the illumination parameters successively at a number of different values, the number being at least equal to the number of Zernike polynomials to be determined;
measuring a same Fourier harmonic for each of said different values, and
calculating the Zernike coefficients from of the measured values for the said Fourier harmonic and by means of stored weighting factors which have been obtained by a previously carried out simulation program.
A single figure is understood to mean a figure having a single contour line which is closed in itself. The contour line is the boundary line between the figure and its ambience.
The method uses the fact that the contour line of a figure having a phase structure is not imaged in a single line but in a first and a second image line, the second image line being located within the first image line, and the distance between the first and the second image line is determined by the point spread function, or Airy distribution, of the imaging system. In the method useful use is thus made of the point spread function, or Airy distribution, of the imaging system. If this system has given aberrations, given deviations of the ideal image occur, such as deviations of the shape of the image lines themselves and/or changes of the mutual position of the two image lines. The method thus allows detection of aberrations which cannot be detected when using a test object in the form of an amplitude, or black-white, structure. When using a test object with an amplitude structure, its contour line is imaged in a single line. Consequently, only the aberrations of the imaging system which cause deviations of the imaged single contour line can be detected when using such a test object, and this even less accurately. When using a test object having a phase structure, different aberrations occurring simultaneously can be detected separately because the effects of the different aberrations remain well distinguishable in the image formed, in other words, the different aberrations do not exhibit any mutual crosstalk. The method uses a Fourier analysis, which operates with sine and cosine functions and is eminently suitable to directly analyze the contour lines of the image. Each aberration, for example, astigmatism is composed of a number of sub-aberrations of lower and higher order. Each of these sub-aberrations are usually represented by a Zernike coefficient, i.e. an amount of a specific Zernike polynomial from the xe2x80x9cfringe Zernike codexe2x80x9d which has a maximum of 37 polynomials. The novel method is based on the insight that the Zernike coefficients of a given aberration can be determined by determining the Fourier harmonic related to this aberration for different illumination conditions. It thus becomes possible to measure the sub-aberrations. Thereby use is made of the linearity of the measuring method.
It is to be noted that, in one embodiment described in U.S. Pat. No. 5,754,299, relating to a method and a device for measuring an asymmetrical aberration of a lithographic projection system, the test object is denoted as phase pattern. However, this pattern is not a closed single figure, but a phase grating, for example, an alignment mark. The image formed of this grating has the same appearance as the grating itself, i.e. each grating line is imaged in a single line. Moreover, for measuring the aberration, an image of the grating is formed every time at different focus settings, and the detection is based on measuring the asymmetries between these images, rather than on detecting changes of shape and/or positions in an image itself.
According to the invention the above-mentioned object can also be met with an alternative method, which comprises the steps of:
arranging a test object, which comprises at least one closed single figure having a phase structure, in the object plane of the system;
providing a photoresist layer in the image plane of the system;
imaging the test object by means of the system and an imaging beam;
developing the photoresist layer;
observing the developed image by means of a scanning detection device having a resolution which is considerably larger than that of the imaging system;
subjecting the observed image to a Fourier analysis in order to ascertain at least one of different types of changes of shape in the image of the single figure, each type of shape change being indicative of a given kind of aberration, which is represented by Fourier harmonics each composed of a combination of Zernike coefficients, and
determining the Zernike coefficients of an observed image by comparing the observed image with an number of reference images, which are stored together with data about their Zernike coefficients in a look-up table, to determine which of the reference images fits best to the observed image, the look-up table having been obtained by a previously carried out simulation program.
The methods are further preferably characterized in that a scanning electron microscope is used as a scanning detection device.
Such a microscope, which is already frequently used in lithographic processes, has a sufficient resolution for this application. Another and newer type of scanning detection device is the scanning probe microscope which is available in several implementations such as the atomic force microscope (AFM) and the scanning optical probe microscope.
The phase structure of the test object may be realized in various ways. For example, the single figure may be constituted by an area in a transparent plate having a refractive index, which is different from that of the rest of the plate.
A preferred embodiment of the novel method is characterized in that every single figure is constituted by an area in a plate located at a different height than the rest of said plate.
Said area may be countersunk in the plate or project from the plate. This plate may be transparent to the radiation of the imaging beam, or reflective.
The single figure may have various shapes, such as the shape of square or of a triangle. A preferred embodiment of the novel method is characterized in that said area is circularly shaped.
The shape of the single figure is then optimally adapted to the circular symmetry of the imaging system, and the image of this figure consists of two circular image lines. A change of the shape and a mutual offset of these image lines can be observed easily. Even if a square single figure is used, the novel method yields good results because the image lines of this figure formed by the projection system are a sufficient approximation of the circular shape.
Each single figure is preferably further characterized in that the height difference between the area of this figure and the rest of the plate is such that a phase difference of 180xc2x0 is introduced in the imaging beam.
For a transmissive, or reflective, test object, this means that the height difference must be of the order of xcex/(2(n2xe2x88x92n1)), or of xcex/4n, in which xcex is the wavelength of the imaging beam, n2 is the refractive index of the material of the test object and n1 is the refractive index of the surrounding medium. At this height difference, the phase difference between the part of the imaging beam originating from the area of the single figure and the part of the imaging beam originating from the surroundings of this area is maximal, and the contrast in the image formed is maximal. If the diameter of the area is of the order of the wavelength of the imaging beam, or of a larger order, the optimal height difference is equal to xcex/(2(n2xe2x88x92n1)) or xcex/4n. At a smaller diameter, polarization effects must be taken into account, and the optimal height difference deviates by several percent from the last-mentioned values.
In accordance with a further preferred embodiment, the diameter of the area is proportional to xcex/(NA.M), in which xcex is the wavelength of the imaging beam, NA is the numerical aperture of the projection system at the image side and M is the magnification of this system.
The size of the test object is then adapted to the resolution of the projection system, allowing measurements of aberrations of the smallest images that can be made with the projection system.
The method may be used, inter alia, for detecting aberrations of a projection system in a lithographic apparatus intended to image a mask pattern, present in a production mask, on a production substrate which is provided with a photoresist layer. This method is further characterized in that a mask having at least a single figure with a phase structure is used as a test object, which mask is arranged at the position of a production mask in the projection apparatus, and in that a photoresist layer with a support is provided at the position of a production substrate.
This method provides the advantage that aberrations of the projection system can be detected under circumstances, which correspond to those for which this projection system is intended. The number of single figures may vary from one to several tens. Since these figures are imaged at different positions within the image field of the projection system, insight is obtained into the variations of the aberrations across the image field. Since the single figures are small, they may be provided in the production mask at positions outside the details of the mask pattern.
However, the method is preferably further characterized in that use is made of an empty test mask having at least a single figure.
The test object is now constituted by a recessed or a raised part of a transparent plate of the same material and having the same thickness as a production mask, but without a mask pattern or parts thereof, which plate may be denoted as empty test mask.
The invention further relates to a system for performing the method described above. The system comprises an optical apparatus of which the imaging system forms part, a test object having at least a single figure with a phase structure, a scanning detection device for scanning at least a test object image formed by the imaging system, and an image processor coupled to the scanning projection device, for storing and analyzing the observed images, and is characterized in that the image processor comprises analysis means for detecting at least one of different types of changes of the shape of said image.
The invention also relates to a lithographic projection apparatus for imaging a mask pattern, present in a mask, on a substrate, which apparatus comprises an illumination unit for supplying an projection beam, a mask holder for accommodating the mask, a substrate holder for accommodating the substrate and a projection system arranged between the mask holder and the substrate holder, which apparatus is suitable for performing the method described above. This apparatus is characterized in that, in the implementation of the method, the projection beam is used as an imaging beam, and in that the illumination unit comprises means for reducing the diameter of the projection beam cross-section for the method to a value which is smaller than the diameter of the projection beam cross-section during projection of the mask pattern on the substrate.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiments described hereinafter.